Index: src/expr.c
==================================================================
--- src/expr.c
+++ src/expr.c
@@ -1846,10 +1846,24 @@
/*
** Walk the expression tree passed as the first argument. Return non-zero
** if the expression consists entirely of constants or copies of terms
** in pGroupBy that sort with the BINARY collation sequence.
+**
+** This routine is used to determine if a term of the HAVING clause can
+** be promoted into the WHERE clause. In order for such a promotion to work,
+** the value of the HAVING clause term must be the same for all members of
+** a "group". The requirement that the GROUP BY term must be BINARY
+** assumes that no other collating sequence will have a finer-grained
+** grouping than binary. In other words (A=B COLLATE binary) implies
+** A=B in every other collating sequence. The requirement that the
+** GROUP BY be BINARY is stricter than necessary. It would also work
+** to promote HAVING clauses that use the same alternative collating
+** sequence as the GROUP BY term, but that is much harder to check,
+** alternative collating sequences are uncommon, and this is only an
+** optimization, so we take the easy way out and simply require the
+** GROUP BY to use the BINARY collating sequence.
*/
int sqlite3ExprIsConstantOrGroupBy(Parse *pParse, Expr *p, ExprList *pGroupBy){
Walker w;
memset(&w, 0, sizeof(w));
w.eCode = 1;